Noble-metal alloys, adapted for ceramic bonding (the application of a porcelain jacket or covering) are well known in dentistry and are used in the manufacturing of crowns, bridges and other prosthetic appliances. These so-called noble "ceramic alloys" typically consist of 80-90% gold, 5-15% platinum and 1-10% palladium. Alloys of this type, and application of the alloys in dentistry, are discussed in detail in U.S. Pat. No. 3,413,723--Wagner & Pralow issued Dec. 3, 1968 and for brevity the disclosure of this patent is incorporated herein by reference.
Due to the increase in price of gold and platinum, alloys with lower gold content were developed, (U.S. Pat. No. 3,667,936, and U.S. Pat. No. 3,981,723). These types of alloys consist essentially of 30-60% gold, 15-50% palladium, and 5-30% of silver. Platinum is limited in use or eliminated from these alloys. A major problem with these known alloys is that they tend to cause greening and discoloration of dental porcelains fused onto the understructure of these alloys, especially for the light shades of porcelains. Metallurgists hypothesize that greening of dental porcelain fused onto these alloys is caused by evaporation and redeposition of silver onto the porcelain or surface diffusion of silver and oxidation of silver or reaction of silver with sulfur during the ceramic firing. Although the exact mechanism is not entirely known, it has been recognized that greening and discoloration of dental porcelain is caused by silver in these alloys. The problem is discussed by R. V. Williams, Jr. et al in Dental Porcelain: The State of the Art-1977, Henry N. Yamada, editor, 1977, University of Southern California School of Dentistry, Los Angeles, California, pages 71-77 and in U.S. Pat. No. 3,667,936.
Coloration of porcelain may be limited by baking one layer of gold powder onto the understructure of the alloy prior to the application of dental porcelain; however, this technique increases cost (gold powder and labor), and causes uncertainty of bond strength. Another way to avoid this problem is the elimination of the use of silver in the alloy and increasing the usage of gold and palladium. This also increases the cost of the alloy.
By adding silicon, boron, germanium or a mixture thereof into the gold-palladium-silver system, I unexpectedly found that this alloy produced extremely clean castings, and this alloy did not cause greening and discoloration of dental porcelain. Addition of these elements in the gold-palladium-silver system caused hot tear in the casting but was eliminated by incorporating a small amount of ruthenium in the alloy.
Williams et al at page 74 of the above publication disclose that they have found traces of silicon in raw materials for gold-palladium base alloy systems but describe the undesirability of such material in the alloy without recognizing that the greening problem they discuss could be eliminated by incorporating a larger amount of silicon in the alloy.